The use of nucleic acids has proved effective for altering the state of a cell. The introduction of deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) into a cell can be used to up- or down-regulate the expression of particular genes in the cell, thereby, impacting one or more biochemical pathways. Of the nucleic acid-based technologies used to alter cell physiology, RNA interference (RNAi) is the general term given for regulating the expression of genes at the post-transcriptional level in diversified organisms. RNAi gene silencing can be accomplished using homologous short (21-23 bp) dsRNA fragments known as short interfering or “siRNA.” When a long dsRNA is introduced into a cell line, the cellular enzyme Dicer will cleave it into short interfering RNA (siRNA) molecules. This short interfering RNA molecule is now called the guided RNA. The guided RNA will guide the RNA-Induced-Silencing-Complex (RISC) to the homologous target mRNA. Once it forms a hybrid structure to the homologous mRNA sequence, the RISC will cleave the mRNA. As a result, protein that is encoded by the mRNA will no longer be produced, thereby causing the silencing of the gene. RNA interference refers to the process of sequence-specific post-transcriptional gene silencing in animals mediated by short interfering RNAs (siRNAs).
However, a major obstacle for the development of RNAi-based therapeutic approaches for brain pathologies is the blood-brain barrier (BBB). The brain is shielded against potentially toxic substances by the presence of two barrier systems: the blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier (BCSFB). The BBB is considered to be the major route for the uptake of serum ligands since its surface area is approximately 5000-fold greater than that of BCSFB. The brain endothelium, which constitutes the BBB, represents the major obstacle for the use of potential drugs against many disorders of the CNS. As a general rule, only small lipophilic molecules may pass across the BBB, i.e., from circulating systemic blood to brain. Many drugs that have a larger size or higher hydrophobicity show promising results in animal studies for treating CNS disorders.
Besides direct intrabrain administration, different strategies have been described for achieving gene silencing in the CNS by means of systemically-administered RNA interfering molecules. For instance, Kumar et al. (Nature, 2007, 448:39-44) have described conjugates of siRNA and a peptide derived from the rabies virus glycoprotein comprising a nonamer arginine and their ability to silence gene expression in the brain after intravenous injection. Xia et al. (Pharmaceutical Research, 2007, 24:2309-2316) have described conjugates comprising a biotinylated siRNA and a conjugate comprising avidin-anti-transferrin receptor antibody which are capable of silencing gene expression in the central nervous system after systemic delivery. WO200979790 describe conjugates comprising siRNA and a series of peptides collectively known as Angiopeps which are capable of crossing the blood-brain barrier by receptor-mediated transcytosis using the low-density lipoprotein receptor-related protein-1 (LRP-1) and which allows the delivery to the CNS of systemically administered conjugates comprising said peptides. WO2007107789 describes the use of compounds capable of causing RNA interference and which are specific for targets present in the CNS and the delivery to the CNS by the use of intranasal administration.
Several reports have speculated about conjugates to synuclein-specific silencing agents and different molecules which might help the translocation of the conjugate across cell membranes or across the blood brain barrier. For instance, WO2011087804 describes conjugates comprising an alpha-synuclein-specific siRNA and a peptide derived from rabies virus glycoprotein G, which allows the conjugate to cross the blood-brain barrier. WO2012027713 describes conjugates of alpha-synuclein-specific dsRNA and different moieties which enhance the activity, cellular distribution or uptake of the dsRNA such as lipid moieties (cholesterol), cholic acid, a thioether, a thiocholesterol, an aliphatic chain (e.g. dodecandiol or undecyl residues), a phospholipid, a polyamine or a polyethylene glycol chain, adamantane acetic acid, a palmityl moiety or an octadecylamine or hexylamino-carbonyloxycholesterol moiety. However, all these conjugates are intended for non-specific delivery across biological membranes or biological barriers but do not confer specificity towards the cells wherein synuclein is expressed.
However, while all these systems allow the delivery of systemically administered siRNAs to the CNS, they do not allow delivery to specific cell types within the brain. WO2011131693 (incorporated herein by reference) discloses conjugates comprising a nucleic acid which is complementary to a target nucleic acid sequence and which expression prevents or reduces expression of the target nucleic acid and a selectivity agent which is capable of binding with high affinity to a neurotransmitter transporter. These conjugates are useful for the delivery of a particular nucleic acid to a cell of interest.
The possibility of delivering siRNAs of known specificity to the central nervous system will be useful for the treatment of diseases which are caused by an undesired activity/expression of a given gene, including depression, cognitive disorders, Parkinson's disease, Alzheimer's disease, etc.
Parkinson's disease (PD) is a degenerative disorder of the central nervous system that often impairs the patient's motor skills, speech, and other functions. The symptoms of Parkinson's disease result from the greatly reduced activity of dopaminergic cells in the pars compacta region of the substantia nigra (SNpc). These neurons project to the striatum and their loss leads to alterations in the activity of the neural circuits within the basal ganglia that regulate movement, in essence an inhibition of the direct pathway and excitation of the indirect pathway. The direct pathway facilitates movement and the indirect pathway inhibits movement, thus the loss of these cells leads to a hypokinetic movement disorder. The lack of dopamine results in increased inhibition of the ventral anterior nucleus of the thalamus, which sends excitatory projections to the motor cortex, thus leading to hypokinesia.
PD is characterized by a progressive loss of dopaminergic neurons in the SNpc and the presence of intracellular inclusions designated as Lewy bodies (LB). Neurochemically, PD is marked by mitochondrial complex I dysfunction and increased indices of oxidative stress. Several pathogenic mechanisms have been proposed for PD including oxidative and nitrosative stress, mitochondrial dysfunction, protein misfolding and aggregation, and apoptosis. PD is mostly sporadic but some of the PD cases have been shown to be familial-linked. The first familial-linked PD gene identified was α-synuclein (α-syn) which in fact is the major component of LB in all PD patients. The normal function of α-synuclein is poorly understood. α-Synuclein can bind to lipids and, in neurons, is associated with presynaptic vesicles and the plasma membrane, possibly via lipid rafts. The deposited, pathological forms of α-synuclein are aggregated and show lower solubility than the normal protein. Three point mutations have been described to cause familial PD, but also duplications and triplications of the SNCA gene have been reported to be responsible for PD and Lewy body disease. Therefore, even without sequence variants, α-synuclein dosage can be causal for Lewy body disease.
α-Synuclein affects mitochondria and probably induces apoptosis. In fact, there is accumulating evidence for a close relationship between α-synuclein and oxidative damage: overexpression of mutant α-synuclein sensitizes neurons to oxidative stress and damage by dopamine and complex I inhibitors, resulting in increased protein carbonylation and lipid peroxidation in vitro and in vivo. Conversely, dysfunction of mitochondrial complex I has been associated to sporadic forms of PD. Complex I dependent oxidative damage and defective mitochondrial function is a main cause of neuronal degeneration and cell death in PD. Thus impaired mitochondrial function and ROS production increases the cytochrome c pool level in the mitochondrial intermembrane space, allowing its rapid release when the cell death agonist Bax is activated.
To sum up, the scenario in PD would be a situation of neuronal mitochondrial dysfunction with increase ROS production that on one hand would increase α-synuclein accumulation and on the other would activate Bax-mediated cell death. Further, α-synuclein accumulation, in turn, would increase cellular ROS production and induction of neuronal degeneration.
The most widely used treatment for PD is L-DOPA in various forms. However, only 1-5% of L-DOPA enters the dopaminergic neurons. The remaining L-DOPA is often metabolised to dopamine elsewhere, causing a wide variety of side effects. Dopa decarboxylase inhibitors like carbidopa and benserazide are also used for the treatment of PD since they help to prevent the metabolism of L-DOPA before it reaches the dopaminergic neurons and are generally given as combination preparations of carbidopa/levodopa and benserazide/levodopa. Moreover, dopamine agonists are moderately effective and act by stimulating some of the dopamine receptors. However, they cause the dopamine receptors to become progressively less sensitive, thereby eventually increasing the symptoms.
Antisense approaches might also be helpful, and have been reported to work in the rat and mouse brain. This approach is predicated on the idea that α-synuclein really is dispensable for CNS function in humans, as it appears to be in the mouse but perhaps even a modest decrease in protein levels would be enough to decrease PD progression.
However, despite the advances made in the development of PD therapeutics, there is still the need of alternative compounds which specifically are capable of preventing the reduced activity of dopaminergic cells in the pars compacta region of the substantia nigra.